The Origin of Oirase and Lake Towada

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○ History of Lake Towada

* The volcano has been enlisted as a Constant Monitoring Volcanoes since December 1st, 2016.

Lake Towada continues to have an active volcano. Towada Volcano started its volcanic activities about 200,000 years ago, and major volcanic eruptions repeatedly occurred from 15,000 ~ about 55,000 years ago. As a large amount of pyroclastic flows were ejected from the eruption, the volcanic body started to cave in. Pyroclastic flows from at least three eruptions pushed the imminent collapse ahead and formed an almost rectangular caldera, the original form of the present-day Lake Towada, and it also released water that made Lake Towada. Later, a huge river channel eroded and the main ground of Oirase Gorge, as you see it now, was formed. Towada Volcano’s last eruption occurred about 1,000 years ago.

○ History of Oirase Stream

Most waterfalls and rock face you see now along the path of Oirase are pyroclastic deposits ejected from the Hakkoda Caldera, 20 km from Oirase, some 760,000 years ago. These are called welded tuff (or ignimbrite) that were large amounts of pumice stones and volcanic ash being accumulated, compressed, and solidified. This pyroclastic flow plateau was eroded by heavy flooding, causing Lake Towada to collapse and forming a deep gorge as an original topographical feature of Oirase Gorge. The outlet for Oirase River from the lake, the present “Nenokuchi,” is also a “place of origin” for Oirase.

Towada Caldera was formed by the collapse and sinking of a magma reservoir during an eruption occurring about 10,000 to 55,000 years ago. Inlet water from the rain and forest, spring water from the lakebed, and other sources of water, built up the caldera that eventually formed Lake Towada. The lake’s water level rose, and a lakeshore collapsed about 15,000 years ago. A huge flood ran down as if scraping off the pyroclastic plateau, making the Oirase River. The level of water in the lake was lowered by the flood, and erosion gradually continued to create what we see today.
 

Nenokuchi as seen from Lake Towada. It clearly shows this very part used to be part of the outer rim of the crater but collapsed, from which the lake water of the caldera flowed out, leading to the forming of Oirase’s deep gorge.
 
*The white fog is a seasonal wind called Yamase flowing from Oirase.